The present invention generally relates to electrical power supplies, and more particularly, to power supplies that employ synchronous rectification.
In flyback and forward switching power supplies, output rectifier power dissipation may be a substantial contributor to the efficiency of a power supply. Synchronous rectification, performed with metal oxide field effect transistors (MOSFETS), may reduce the power dissipation of the rectifier and improve efficiency. As compared with diode rectification, synchronous rectification may also allow a power converter of the power supply to operate at small load currents with lower output ripple voltage because a “dead region” of the diodes may be eliminated. The challenge of synchronous rectifier circuits is to provide the correct timing between a catch MOSFET and a forward MOSFET. Small changes in timing may significantly change the efficiency of the power supply. If gate drives are active simultaneously, cross conduction will occur. If the gate drive timing is too slow, the advantage of synchronous rectification is reduced because a body diode and/or a parallel diode may conduct and dissipate power.
Consequently, many synchronous rectifiers are constructed so that a time delay between operations of the MOSFETS is made low, but not so low as to increase the risk of cross conduction. In this context, it is important to be mindful of the temperature range in which the rectifier may operate, because MOSFETS, due to their inherent parasitic capacitance, may exhibit variations in timing as a function of their temperature. In other words, a particular MOSFET may exhibit a first response time to a gate driver at a low temperature and a different response time to the gate driver at a high temperature. These potential temperature-related response time variations may be significant in rectifiers which may be exposed to wide temperature ranges. For example, a rectifier in an aircraft at ground level may be at a temperature as high as 120° F. The same rectifier may be exposed to temperature as low as −70° F. when the aircraft is in flight.
Additionally, a particular manufactured lot of MOSFETS may exhibit timing characteristics which may be different from timing characteristics of a different lot of MOSFETS. In other words MOSFETS may exhibit lot-to-lot timing variations when incorporated in synchronous rectifiers.
Conventional synchronous rectifiers are constructed with the timing of the gate drivers established so that lot-to-lot variations and temperature-related variations of response time of the MOSFETS do not allow cross conduction to occur. For example, if it is empirically determined that a particular type of MOSFET may have a range of possible lot-to-lot and temperature-related response time variations of up to T nanoseconds (ns) for a temperature range between +120° F. and −70° F., then a rectifier constructed to operate within those temperature limits may incorporate an extra delay time of T ns or more to assure that cross conduction does not occur. This extra delay time has the effect of reducing the overall efficiency of the rectifier.
As can be seen, there is a need for a power supply with a self powered synchronous rectifier that may be constructed with delay times that are independent of lot-to-lot and temperature-related timing variations of MOSFETS.